1. Field of the Invention
The present invention relates to a communication channel setting method, a communication controller, and a radio communication system, which set a communication channel between radio base stations.
2. Description of the Related Art
As a technique of realizing a full duplex communication between radio stations in a radio communication system, there is known a frequency division full duplex communication, which uses different frequencies for a transmission line and a reception line.
In such a conventional radio communication system, for example, as illustrated in FIG. 1, the frequency band used for radio lines is divided, for example, into two frequency bands, i.e. a low frequency group and a high frequency group with a guard band which is set in between. And, it is arranged that a frequency band is assigned to a radio line for transmission (transmission line) and to a radio line for reception (reception line) by paired channels which correspond to the low frequency group and the high frequency group, respectively. In FIG. 1, the symbols A to E and the symbols A′ to E′ each represent a radio line, and each arrow represents a band which is assigned to each radio line. The A and A′, B and B′, etc. each represent paired channels in the low frequency group and the high frequency group which are used for the transmission line and for the reception line, respectively.
In addition, in the conventional radio communication system, the bandwidth of the frequency band (the bandwidth of the channel) which is assigned to each radio line was made fixed regardless of the amount of information which was forwarded to the radio line.
However, in the conventional radio communication system, it is common that the frequency for the channel for the transmission line and the frequency for the channel for the reception line were managed as one pair having a fixed relationship (e.g. the frequency for the transmission line=the frequency for the reception line+the shifted frequency) in order to simplify the configuration of the radio station, etc.
In this case, only in a case where both the channel for the transmission line and the channel for the reception line are usable, can these paired channels be used as communication channels. Therefore, as illustrated in FIG. 2 for example, in a case where any one of the channels cannot be used, that pair cannot be used. That is, although in each of a channel X1 and a channel X3 the received power of the interference wave is smaller than a prescribed threshold and can be used as the transmission line in the band of the low frequency group, only a channel X3′ can be used as the reception line in the band of the high frequency group due to the level of received power of the interference wave. The result is that the channel X1 cannot be used as the transmission line and therefore only paired channels X3 and X3′ can be used as the communication channels.
In a case where, as in the case of a network for radio base stations, a number of radio lines exist on the same plane, since in many cases the interference which the transmission radio line receives and that the reception radio line receives become asymmetric due to fading having frequency dependency, for example, multi-path fading, and a directional antenna used at the base stations, the assignment of a channel pair by the method as mentioned above is not able to achieve an effective assignment. This raises the problem that frequency utilization efficiency of the system is decreased.
Further, in the above-described radio communication system, since the acquisition of the bandwidth of the channel is fixed, the channel always occupies the entire prescribed bandwidth even when the amount of information to be forwarded is very small. For this reason, there was the need to improve the method for the channel assignment to enhance frequency utilization efficiency.